While metal oxide catalysts have held an eminent role in industrial scale reactions for the selective oxidation of hydrocarbons, there remains a desire to better understand the correlation of molecular structure and chemical nature in these systems. Newer processes that utilize metal oxides, such as oxidative dehydrogenation and direct valorization to oxygenates, require improvements in selectivity, productivity, and stability in order to fulfill their promise as viable improvements over more classical methodologies. And, among the transition metal oxide catalysts implemented for the production of olefins and oxygenates, those containing group-V elements, and even more so, those containing vanadium, have demonstrated the most potential. Recently, we have employed a novel spectrokinetic approach with Raman spectroscopy to gain unique insights on the structure-reactivity relationships of silica supported vanadium oxide catalysts. We define spectrokinetics as an operando spectroscopic methodology in which reaction rates (kinetics) are obtained directly from spectroscopic data. In prior seminal work, redox reactions with a series of ternary metal oxide catalysts were time-resolved using pulsed injections of gaseous reagents while following by mass spectrometer and monitoring the evolution of the (dispersed) vanadyl Raman transition. Reaction rates were obtained from the Raman-spectrokinetic data, as well as the MS data, which enabled us to distinguish the contribution of a specific active site (the V=O evolution) from the overall bulk oxidation rate (determined via MS), which, coupled with the surface structure information afforded by the entire Raman spectra, provided insights into synergistic effects in the multicomponent systems studied that were further corroborated by H2-TPR data. With our current work, we endeavor to improve upon the Raman-spectrokinetic approach by studying a series of V-SiO2 catalysts at different metal loadings containing both 2D (dispersed) and 3D (nanoparticle) vanadium species, providing two distinct Raman transitions that can be monitored (998 and 1032 cm-1). Tracking these two signals during reduction and oxidation reactions, unique rate information from two separate surface species was obtained. The Raman-spectrokinetic experiments were then coupled with XRD experiments to elucidate how the size of the particles present further impacted the redox properties.

New insights on the redox properties of two- and three-dimensional supported vanadium oxide catalysts: A Raman-spectrokinetics study

2019 Spring National Meeting

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